As driving assistance control executed by a vehicle driving on a dirt road which is an unpaved road surface, vehicle downhill control such as DAC (Downhill Assistant Control) is known. In the vehicle downhill control, a brake actuator is actuated to prevent a vehicle body speed of the vehicle from exceeding a target speed (for example, 5 km/h) so as to adjust braking force to be applied to the vehicle.
Even though the vehicle downhill control is executed in a vehicle driving on a downhill slope, the driver of the vehicle may execute an accelerator operation. In this case, in a driving assistance device described in Patent Literature 1, a target speed is set to increase in accordance with an increase in accelerator operation amount by a driver. In this manner, even in execution of vehicle downhill control, a vehicle body speed of the vehicle can be increased depending on the intention of the driver who executes the accelerator operation.
When a downhill slope on which a vehicle drives is steep, a decrease in braking force of the vehicle may cause the vehicle to be rapidly accelerated under the influence of gravity acting on the vehicle. For this reason, in execution of vehicle downhill control, even though a driver executes the accelerator operation to increase the vehicle body speed of the vehicle, the accelerator operation amount may be very small. In this case, even though the driver executes the accelerator operation, the vehicle body speed of the vehicle is difficult to increase because the target speed does not increase.
The vehicle downhill control may be executed not only when the vehicle drives on a sloping road but also when the vehicle drives on a snowy road. Even though the driver executes the accelerator operation in execution of the vehicle downhill control when the vehicle is driving on a snowy road, the accelerator operation amount may be very small. In such a case, even though the driver executes the accelerator operation as in driving of the vehicle on a downhill slope, the vehicle body speed of the vehicle is difficult to increase because the target speed does not increase.